def numRabbit(mountNum):
    if mountNum == 1:
        return 2
    if mountNum == 2 :
        return 3
    return numRabbit(mountNum-1) + numRabbit(mountNum-2)


if __name__ == '__main__': 
    testRabbit = eval(input(""))
    count = numRabbit(testRabbit)
    print(count)